The only theory like that I have heard was made up by myself actually. One day a while back I just had the realization that oil could be the equivalent of the blood of the earth....and those greedy humans are sucking all the blood right out of mother earth.

Seriously though, it probably did serve some type of function that we may or may not even be able to comprehend. It could also just have been a simple function such as insulating the surface of the earth from the molten core...who knows?